PTL 1 discloses a configuration in which a master router and a backup router are arranged in parallel. According to PTL 1, a shared hub arranged upstream the parallel routers duplicates a packet and transmits the duplicated packets to the master router and the backup router, respectively. While the master router forwards the packet, the backup router drops the packet. In this way, a state can be synchronized without performing communication between the master and backup routers.
PTL 2 discloses a packet transfer apparatus that duplicates a packet therein. The duplicated packets are forwarded to path control modules in active and standby systems, respectively, in the packet transfer apparatus and these path control modules perform path control processing. The packet transfer apparatus determines a state of the standby module by receiving an internal control packet from the standby module.
NPL 1 proposes a method for reducing state exchange overhead by duplicating a packet and forming a redundant state.
NPLs 2 and 3 relate to a centralized-control-type network referred to as OpenFlow.